A typical plug board system of the prior art is shown in U.S. Pat. Nos. 3,348,187, or 3,341,801, and is of the type for programming the electrical in and out, IO, connections of a machine such as a computer or data collection terminal. Such a plug board system comprises a rear bay having an apertured rear board into which are plugged electrical connections from a machine to be programmed. The plug board system further includes a separate front bay having another apertured board on which interconnections are made by a patterned array of patchcords, typically described in U.S. Pat. No. 3,781,763. As the front bay is assembled on the rear bay the patchcord interconnections become plugged into the apertured rear board whereupon they contact and thereby interconnect the IO connections. By changing the pattern of patchcord interconnections on the front board, the interconnections to the apertured rear board can be changed. The program changes therefore appear only on the front board leaving unchanged the interface of the rear board and the machine to be programmed.
Sometimes it is desired to add circuit changes or circuit modifications which are more complex than mere patchcord connections. For example, it may be desirable to modify incompatible or mismatched circuits before interconnection thereof with patchcords. Other circuit modification include, signal conversion, voltage or power control, signal or wave modification, etc. Other circuit conditions requiring modification may exist but are too numerous and varied to be set out in detail. It is sufficient to mention that patchcord programming systems of the prior art have been inadequate to accommodate circuit modifications requiring something more than patchcord interconnections.
The circuit modifications being considered are those that require active circuit devices on circuit boards which sometimes are incorporated internally of the machine being programmed. This, however, requires entry of the machine being programmed. Often it is not practicable to enter the machine and modify it internally. The only other choice is to mount the modifying circuit boards externally of the machine and provide wire or cable interconnections running to the machine and also to the plug board. Such interconnections are lengthy and unwieldy. Oftentimes the plug board directly covers the IO panel of the machine to be programmed. The addition of modifying circuit boards then requires relocation of the plug board and reconnection of the same to the IO panel and to the modifying circuit boards by electrical conductors or cables.
What has been described are the problems associated with incorporating circuit modifications into a machine being programmed by a plug board system. Due to the complexity and the number of circuit modifications which are desired in order to make the programmed machine more versatile, a number of circuit modifying devices are required. Multiple circuits are usually fabricated into plug in control modules. More specifically, these modules are in the form of active circuit devices interconnected by plated metal circuit paths on flat circuit boards which are commonly called cards. The cards are typically of common size and shape and are contained within an enclosure commonly called a card cage. The cards are arranged in parallel spaced relationship with the edges of the cards projecting from the card cage. Electrical connections are made to these card edges and thereby to the circuit paths and active circuit devices. A problem exists whenever the card cage is to be electrically connected to the machine to be programmed and also to a plug board which normally programs the machine. For example, the machine to be programmed is itself fabricated of one or more card cages. There is usually no room in the machine for additional card cages. This and other considerations exist which require the added card cage to be mounted externally of the machine. The impractical result is an unwieldy array of electrical wires running between the machine to be programmed, between the card cage and the plug board, and between the machine and the plug board.